Interleukin 4 (IL-4) is potent lymphokine that regulates a wide variety of functions of lymphoid and other hematopoietic cells. An assay to measure IL-4 production by single cells indicates has been developed. It shows that the frequency of T cells capable of producing Il-4 in lymph node cell suspensions from normal mice very low, about 1/1000, is found mainly among large T cells and is completely dependent upon the presence of interleukin- 2 (IL-2). Lymph node T cells from mice that had been injected with antiIgD or infected with Nippostrongylus brasiliensis, treatments known to cause an IL-4-dependent increase in serum IgE, show a frequency of IL-4 producing cells five to ten times as great as T cells from naive donors. Although small resting T cells from naive donors have a frequency of IL-4 producing of about 1/2000, they can be "primed" in vitro to develop into IL-4 producing cells by culture with anti-CD3, IL-2 and IL-4; this increases the frequency of IL-4 producing cells to about 1/20. Both IL-2 and IL-4 are required for this in vitro priming method and interferon gamma fails to inhibit such priming. IL-4 is also made by long term mast cell lines in response to receptor cross-linkage, as are other members of the IL-4 family of lymphokines, notably IL-5, IL-3, and GM-CSF. Among normal spleen and bone marrow cells, a small population of cells have high affinity receptor for IgE. These Fcepsilon receptor positive cells have essentially all the IL-4 producing activity of spleen cells in response to high affinity IgE receptor cross- linkage; frequencies of IL-4 producing cells as high as 1/5 have been achieved. Many of these cells are Alcian Blue positive suggesting that they are either mast cells or basophils. Cells capable of forming mast cell colonies in methylcellulose have a frequency of about 1/10 in this cell population. On a per cell basis, the Fcepsilon receptor positive splenic cells appears to make more than 100-fold more IL-4 per cell than do cells from IL-3-dependent mast cell lines, suggesting that they may be important cells in the mediation of IL-4 dependent events in vivo.